1. Technical Field
The present invention relates to directional microphone array systems and methods to calibrate directional microphone array systems.
2. Discussion of the Related Art
Directional microphone systems may be used in conjunction with high-fidelity audio systems to record and reproduce acoustic signals having directionality, such as signals originating from different locations. Examples of signals having directionality include an aircraft flying overhead, different instrumental sections at different locations in a large orchestra, sounds originating from different players on a sport field and sounds from spectators. The recording and reproduction of acoustic signals having directionality can improve the realism of the reproduced sound field for the benefit of the listener.
A directional microphone system used to detect acoustic signals having directionality can comprise a microphone array and associated electronics for digital processing of the detected signals. Some of these systems delay and subtract the multiple microphone signals in a method known as differential microphone technique. (See, G. W. Elko, “A Simple Adaptive First-order Differential Microphone”, Air-Coupled Acoustic Microsensors Workshop (1999)) In some applications, digital processing and differential microphone techniques are used to acquire B-format signals, which consist of three coincident signals: an omnidirectional signal and two dipole (figure-of-eight) signals with polar directivity pattern that point to the front-back and left-right directions. These signals can be acquired from a low-cost, closely spaced omnidirectional microphone array comprising at least three microphones arranged in a two-dimensional configuration. The B-format omnidirectional signal can be acquired from any one microphone in the array. The two dipole signals can be acquired by differential microphone techniques using plural microphones in the array.
To produce B-format polar directivity patterns, e.g., for the dipole signals, responses of the microphones in the array should be closely matched in terms of amplitude response and phase response. One method for matching responses of the microphones is to measure and sort the microphones manually during manufacturing so as to select sets of microphones wherein each microphone in a set has a response closely matched to responses of other microphones in the set. Another method is to run a calibration routine during assembly, and digitally compensate the mismatches via digital filtering embedded in the platform where the microphone array is to be used.
The terms used herein referring to matching the responses or equalizing the responses of microphones are used in reference to an ideal condition in which plural microphones receiving an identical acoustic disturbance produce substantially identical responses, e.g., substantially identical electrical signals at all response frequencies.
Equalization of the microphone responses in an array with adaptive filtering is also possible, for example, by using one of the microphones in the array as a desired or reference signal, and adapting all the other microphone's signals according to the reference signal. (See, M. Buck, T. Haulick, H. Pfleiderer, “Self-calibrating microphone arrays for speech signal acquisition: A systematic approach”, Signal Processing 86, pp. 1230-1238, Elsevier (2006)) An example of an adaptive filtering unit 100 is shown in FIG. 1, which includes delay element 110. A detected signal xIC,μ(k) (for example, from one of the microphones in the array) is matched to the reference signal dIC,μ(k) by an adaptive filter 120.
Output signals from the unit of FIG. 1 may include a calibrated signal xCIC,μ(k) and an error signal eIC,μ(k). The error signal can be used to update adaptive filter coefficients in filter 120 such that the difference between the reference signal, or a delayed version of the reference signal, and the calibrated signal is minimized. In this manner, the calibrated signal xCIC,μ(k) can be approximately matched to the reference signal dIC,μ(k).
FIG. 2 shows a possible embodiment of microphone self-calibration for multiple microphones in a microphone array. The filter blocks 210 correspond to the filtering unit depicted in FIG. 1. In the embodiment of FIG. 2, each microphone in the array may be matched to a reference microphone, which may be an independent reference microphone not in the array or any one of the microphones in the array. Many conventional adaptive microphone calibration methods work in this manner, i.e., using adaptive filtering to match the response of each microphone in an array to a selected reference response. The correction of microphone responses may be carried out once during assembly of a device incorporating the microphone array.
Manual sorting and grouping of microphones with similar responses can be time-consuming and labor-intensive. Further, due to component aging and other mechanical factors, matched responses of microphones in a set is not guaranteed over the long term. Similarly, running calibration and digital compensation procedures during assembly of a microphone array platform can also be time-consuming and require expensive measurement and calibration equipment. Additionally, due to aging and/or packaging of the array, microphone responses may change over time and the initial calibrations become obsolete. Re-calibrations would then be required.
The inventors have contemplated that adaptive calibration techniques can be useful in directional microphone array systems when implemented as self-calibration methods that can be executed by the system repeatedly over the lifespan of the system. The inventors have recognized that previous techniques for calibrating microphones may not be suitable for use over the lifetime of a device due to the expensive calibration equipment needed and/or time or cost required to run a calibration procedure. The inventors have also recognized that adaptive filter compensation systems like those depicted in FIG. 2 may not accurately match microphone responses, since these systems typically compensate for magnitude of the acoustic signals, but not the phase. For example, when the microphones in the array are spaced apart at various distances, the phase of the signals in each microphone may be different depend on the incoming sound direction. In an uncontrolled environment, the adaptive filter may equalize the magnitude properly, but not the phase.